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How Special Is Your Child? April 14, 2009

Posted by federalist in Uncategorized.

My wife wondered, “What is the probability of a couple having two children that are genetically identical (excepting twins)?”  Initially I figured this would normally just be 223 * 223.  The unique genome of a normal human consists of 46 chromosomes (22 homologous pairs, and two sex chromosomes), half of which come from each parent.  During normal meiosis each gamete (i.e., sperm or egg) receives one of each 23 parent chromosome pairs at random, and during conception two gametes are combined to form the zygote.  Hence the odds of two separate zygotes getting the same chromosome copies is 1:246 = 1:70 trillion. (This excludes the relatively rare cases of mutation and abnormal meiosis, each of which can further increase the universe of unique and viable genotypes that a single couple can produce.)

But molecular biology is never that easy. It turns out that the probability of having genetically identical children (unless they are formed as identical twins due to the splitting of a zygote within a few days after conception) is infinitesimal: The process of meiosis not only involves the random allocation of chromosome pairs to each gamete, but also exposes each chromosome to a phenomenon known as crossover which allows specific genes from one chromosome to switch places with its pair.  Hence, a child could end up with unique autosomal chromosomes that do not match any of his parents’.  (The probability of crossover depends on characteristics of the genome itself, and is statistically described using map units.)

Normally every one of a child’s genes will match one of his parent’s.  But the variations introduced by normal meiosis make it all but impossible for two separately-conceived children of the same parents to be genetically identical.


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